There was a mix of ethnicities in every unit. The only exception I can think of is the battle for Brest Fortress, where the existing command structure collapsed due to disorganization and unpreparedness for attack, and ethnically oriented units formed organically. If you speak Russian, you can get more details in this video

Ethnic breakdowns of other units are available, not at platoon level, but here's one for the 60th Army, for example.